This invention relates to an optical system and method for calibrating a set of filters forming a part of a space-borne imaging system employing a scanning mirror and, more particularly, to the resolution of solar radiation into the spectral colors, as by use of a prism or grating, and a scanning of the spectral colors by the scanning mirror past the filters to continually update color-transmissivity profiles of individual ones of the filters.
There is considerable interest in the use of spacecraft for photographing terrain of the earth, such as by use of an imaging system which views the terrain through a plurality of filters which identify spectral components, or colors, of various features in the terrain. For example, colored images of farmland can be used to identify crops which are suitably watered as compared to crop which may be suffering from a lack of water. The quality of data obtained from such a photographic survey of the earths terrain is dependent, in large measure, upon the quality of the color filters employed in the photographic system.
An aspect of color filters of considerable concern is the fact that their characteristics may vary in time. For example, a yellow filter which has high transmissivity to the color yellow while attenuating both orange and green colors might tend to drift during extended use upon a spacecraft such that the filter function may shift to allow passage of a significant amount of orange along with the yellow, or a significant amount of green along with the yellow. Such shifts in the filter function may be characterized by a shift in the frequency or wavelength at which a peak transmissivity occurs, or in a broadening of the peak, or by reduced attenuation of colors distant, in terms of spectral distribution, from the color of peak transmissivity. In any event, such a shift in the functions of one or more of the color filters must be known by scientists evaluating the photographic data obtained by the spacecraft and transmitted from the spacecraft back to a receiving station on the earth. Updated information of the filter functions permits scientists to compensate for shifts in the filter functions so as to enable an accurate interpretation of the photographic images. However, in the absence of some form of filter calibration during operation of the spacecraft, there may be a degradation of the accuracy of the color data due to aging of the filters with consequent changes in the color-attenuation profiles of the filters.